There is known a method for preheating steel scrap to a prescribed temperature with the use of high-temperature waste gases produced in a steel-refining metallurgical furnace, for example, an electric furnace when manufacturing steel from the steel scrap as raw material in the steel-refining metallurgical furnace. According to this method, it is possible to reduce the refining time in the electric furnace, and thus save the electric power consumption required for refining.
FIG. 1 is a schematic sectional view illustrating an embodiment of the conventional apparatus for preheating steel scrap by the above-mentioned high-temperature waste gases. In FIG. 1, 1 is a preheating vessel for receiving and preheating a steel scrap, which vessel 1 has cylindrical drum portion 1b provided with an open top 1a, and a two-part bottom lid 2,2 which is movable by means of a suitable opening/closing device (not shown). This preheating vessel 1 is adapted to receive steel scrap to be preheated from the open top 1a thereof, and to discharge the preheated steel scrap by opening the bottom lid 2,2.
Also in FIG. 1, 7 is a hood downwardly flaring for covering the open top 1a of the preheating vessel 1. The top end of the hood 7 is provided with a waste-gas inlet 8 to which a blow pipe 9 of high-temperature waste gases is connected. The hood 7 is movable from the open top 1a of the preheating vessel 1, and is adapted to introduce the high-temperature waste gases into the preheating vessel 1.
In FIG. 1, 4 is a pit for housing the preheating vessel 1. The pit 4 is provided in the ground, and has a depth capable of containing substantially half the height of the preheating vessel 1; 10 is a duct opening at an end thereof to the lower part of the side wall of the pit 4, the other end of the duct 10 being connected to a chimney through a fan and a dust collector, and the waste gases after preheating the steel scrap contained in the preheating vessel 1 are discharged from the pit 4 through the duct 10; 5 are a plurality of stands installed on the bottom of the pit 4 for supporting the preheating vessel 1; and, 6 is a sealing water groove provided on the ground surrounding the pit 4.
In FIG. 1, 3 is a canopy provided along the outer peripheral wall surface of the drum portion 1b of the preheating vessel 1 substantially at the middle of the height of the drum portion 1b for sealing a gap between the pit 4 and the preheating vessel 1 placed in the pit 4. The outer peripheral edge of the canopy 3 is provided with a downwardly extending flange 3a which is inserted into the sealing water groove 6 when the preheating vessel 1 is placed into the pit 4, and thus the gap between the outer peripheral edge of the canopy 3 and the pit 4 is liquid-tightly sealed.
When preheating steel scrap in the conventional apparatus described above, the preheating vessel 1, containing the steel scrap to be preheated is placed on a plurality of stands installed on the bottom of the pit 4, and at the same time the flange 3a of the canopy 3 fixed to the drum portion 1b of the preheating vessel 1 is inserted into the sealing water groove 6. Thereafter, the open top 1a of the preheating vessel 1 is covered with the hood 7, and then, high-temperature waste gases are introduced from the hood 7 into the preheating vessel 1.
The high-temperature waste gases introduced from the hood 7 into the preheating vessel 1 pass through the preheating vessel 1 as shown by the arrows in FIG. 1 while preheating the steel scrap contained in the preheating vessel 1. The waste gases after thus preheating the steel scrap are discharged into the pit 4 through the juncture of the bottom lid 2,2 and a gap present in the contact portion between the bottom lid 2,2 and the drum portion 1b. Since the gap between the pit 4 and the preheating vessel 1 is sealed by the canopy 3 fixed to the drum portion 1b of the preheating vessel 1, the waste gases discharged into the pit 4 after preheating the steel scrap are totally withdrawn through the duct 10, and discharged to open air from the chimney through a dust collector.
The above-mentioned conventional apparatus is problematic in that, when covering the open top 1a of the preheating vessel 1 with the movable hood 7 for preheating steel scrap, gaps are produced in the contact portion between the open top 1a of the preheating vessel 1 and the hood 7, through which gaps the ambient air comes into the preheating vessel 1. Such gaps are caused by the insufficient shape accuracy and thermal deformation of the end surface of the open top 1a of the preheating vessel 1 and the inner peripheral surface of the hood 7 in contact with this end surface of the open top 1a, or caused by defectively attaching the hood 7 to the preheating vessel 1.
The ambient air coming into the preheating vessel 1 causes a temperature drop of the high-temperature waste gases blown from the hood 7 into the preheating vessel 1, thus resulting in an insufficient preheating of the steel scrap. Furthermore, if the ambient air comes into the preheating vessel 1, the amount of waste gases which must be withdrawn through the discharging duct 10 is increased by this air ingression, thus requiring installation of an unnecessarily large-capacity fan for sucking waste gases and an unnecessarily large-capacity dust collector for collecting dust from the waste gases.
Under such circumstances, there is a strong demand for the development of an apparatus for preheating steel scrap by introducing high-temperature waste gases discharged from a steel-refining metallurgical furnace into a preheating vessel containing steel scrap to be preheated, and preheating the steel scrap, in which apparatus the ambient air does not come into the preheating vessel and which apparatus permits efficient preheating of the steel scrap to a prescribed temperature. But such an apparatus is not as yet proposed.